ACTG 320 was a clinical trial of anti-HIV medications which compared the use of a three drug combination Zidovudine (ZDV)+Indinavir(IDV) to a two drug combination (ZDV + 3TC), in persons who ahd previously taken Zidovudine ZDV. The three drug combination included two medications that were potent and new, as compared to the two drug combination which had only one new drug. This study showed that the three drug combination was more effective.